Formerly, the hot rolled steel sheet was not applied to the reinforcing members of automobile cabin from the viewpoint of its poor formability. In recent years, however, the increasing need for steel sheets having low cost and high formability has encouraged the study on the application of the inexpensive hot rolled steel sheet to these members. In particular, the hot rolled steel sheet which is inferior in the surface property to the cold rolled steel sheet is suitable for these inner members. Although there are increased uses of high strength hot rolled steel sheets having a tensile strength of 440 to 590 MPa to crashworthiness members such as a front side member of automobile, higher strengthening of these high strength hot rolled steel sheets is desired.
The hot rolled steel sheet to be applied to these members is required to have a high tensile strength of 780 MPa or more and excellent elongation and stretch-flangeability. Particularly, the hole expansion ratio, which is a criterion of the stretch-flangeability, should be 60% or more.
For improving the elongation, JP-A-7-62485, (the term “JP-A” referred to herein signifies “Japanese Patent Laid-Open Publication”), proposes a dual phase steel sheet in which hard second phase of residual austenite is dispersed in a matrix of ferrite. The steel sheet, however, does not have excellent stretch-flangeability because of the large difference in hardness between the matrix of ferrite and the second phase of residual austenite.
JP-A-9-263885 provides a dual phase steel sheet of which the elongation and the stretch-flangeability are improved by precipitation hardening the matrix of ferrite to decrease the difference in hardness between the matrix of ferrite and the second phase of martensite. The steel sheet, however, gives a tensile strength below 780 MPa, and therefore is not suitable for the reinforcing members of automobile cabin or the crashworthiness members of automobile.
As a dual phase steel sheet having a tensile strength of 780 MPa or more, JP-A-5-179396 proposes a steel sheet having the stretch-flangeability improved by precipitation hardening the matrix of ferrite and decreasing the volume fraction of the second phase of martensite or residual austenite. Although the carbon equivalent of the steel sheet is decreased to improve the spot-weldability and the fatigue characteristic, the hole expansion ratio is at most 46%, which does not give sufficient stretch-flangeability for the reinforcing members of automobile cabin and the crashworthiness members in complex shape of automobile.